The present invention is directed to safety barrier circuits for preventing potentially dangerous electrical threats from entering an adverse environment over electrical conductors which convey analog signals between the adverse environment and an environment outside the adverse environment, and more particularly, to an active safety barrier circuit of the optical isolation type for passing analog signals over an electrical conductor across the barrier between the adverse and outside environments without affecting substantially the accuracy thereof.
Adverse environments are areas, like aircraft fuel tanks, for example, in which sufficient concentrations of reactive materials are present in a state to potentially cause an explosive reaction pending the introduction of an ignition source. Thus, an ignition source would be considered a dangerous threat to such an environment. Examples of potential electrical ignition sources are high voltages, lightning, high intensity radio frequency signals, and the like. Safety barriers circuits prevent these potentially dangerous threats from propagating through electrical conductors which pass signals to and from the adverse environment across a barrier, such as a fuel tank wall, for example.
Some types of safety barrier circuits use resistance in series with the electrical conductor and transient absorbing devices, like high powered zeners, for example, coupled between the electrical conductor and ground potential. Typically, adverse environments may undergo large temperature swings which affect the resistance of these safety barriers. On an aircraft, for example, areas outside of the pressurized vessel where the fuel tanks are disposed may undergo temperature variations from xe2x88x9265xc2x0 C. to +85xc2x0 C. This temperature induced change in resistance may create errors in the measurement signals being passed by the safety barriers. In addition, the transient absorbing devices have a tendency to inject noise in the ground loop of the measurement which may cause overall system type errors. Accordingly, this type of safety barrier circuit is not well suited for passing sensing and measuring signals requiring high accuracy.
Another type of safety barrier circuit is the optical isolator type in which a light source and light detector are disposed in series with the electrical conductor. An electrically non-conducting material, like glass, for example, is disposed between the source and detector for passing light therebetween while creating a barrier which prevents potentially dangerous electrical threats from passing into the adverse environment over the electrical conductor. However, optical isolators have inherent non-linear transfer characteristics which are substantially altered with variations in temperature. This trait of optical isolator type safety barrier circuits render them useless for passing analog signals accurately.
Some system designers have proposed converting an analog measurement signal into a digital time based signal of a single width modulated pulse or a series of start and stop pulses having a modulated interpulse period. An example of such a system is disclosed in U.S. Pat. No. 4,963,729, entitled xe2x80x9cOptically Powered Sensor System with Improved Signal Conditioningxe2x80x9d, and assigned to the same assignee as the instant application. In either case, the changing transfer characteristics of the optical isolator with temperature may alter the shape of the pulse being passed thereby and thus, introduce an error in the time based measurement. In addition, the process for converting the analog signal or measurement into timing signals may cause a greater response time for the measurement readings and ultimately, affect the accuracy thereof.
In some adverse environments, especially those which are enclosed, like an aircraft fuel tank, for example, fuel sensing and measuring equipment and the interconnecting cabling therefor are disposed throughout the tank and aircraft before the enclosure thereof. Once enclosed, it is difficult to make modifications within the tank and aircraft. Accordingly, it would be of paramount importance to retain the existing sensing and measuring equipment and the interconnecting cabling in the performance of any retrofit for adding or improving immunity of these adverse environments to electrical ignition sources.
In accordance with one aspect of the present invention, an active safety barrier circuit is disposable in series with an electrical conductor at a barrier between an adverse environment and an outside environment for passing an analog signal over the electrical conductor across the barrier while preventing potentially dangerous electrical threats from entering the adverse environment over the electrical conductor. The safety barrier circuit comprises: an analog-to-digital converter circuit for converting sequential time samples of the analog signal to a train of electrical digitally coded words representative thereof, the electrical digitally coded words of the train being output sequentially from the analog-to-digital converter circuit; an optical isolator coupled to the analog-to-digital converter circuit for converting the electrical digitally coded words output therefrom into light signals representative thereof which are passed across an electrically non-conductive barrier, and for reconverting the light signals back into the electrical digitally coded words after crossing the non-conductive barrier, the reconverted electrical digitally coded words being output sequentially from the optical isolator, the non-conductive barrier for preventing potentially dangerous electrical threats from entering the adverse environment over the electrical conductor; and a digital-to-analog converter circuit coupled to the optical isolator for reconstructing the analog signal from the reconverted electrical digitally coded words being received sequentially from the optical isolator without affecting substantially the accuracy thereof.
In accordance with another aspect of the present invention, a system for measuring a parameter inside an adverse environment with a sensor disposed inside the adverse environment includes electrical circuitry disposed in an environment outside of the adverse environment for receiving an electrical analog measurement signal from the sensor over a first electrical conductor. The adverse environment is susceptible to a combustible reaction from dangerous electrical threats received from the outside environment over the electrical conductor. The system comprises a first active safety barrier circuit disposed in series with the first electrical conductor at the barrier between the adverse environment and the outside environment for passing the electrical analog measurement signal from the sensor to the electrical circuitry over the first electrical conductor while preventing potentially dangerous electrical threats from entering the adverse environment over the first electrical conductor.
In accordance with yet another aspect of the present invention, the system further comprises: electrical circuitry disposed in the outside environment for generating an excitation signal which is conducted to the sensor over a second electrical conductor, the adverse environment susceptible to a combustible reaction from dangerous electrical threats received from the outside environment over the second electrical conductor; and a second active safety barrier circuit disposed in series with the second electrical conductor at the barrier between the adverse environment and the outside environment for passing the excitation signal across the barrier to the sensor over the second electrical conductor while preventing potentially dangerous electrical threats from entering the adverse environment over the second electrical conductor.